Semiconductor integrated circuit having built-in PLL circuit

ABSTRACT

A semiconductor integrated circuit having a built-in PLL circuit which has two charge pump circuits for charging and discharging capacitive elements of a loop filter in response to signals generated by a phase comparator circuit. One of the two charge pump circuits has current sources which generate current values smaller than those generated by current sources of the other charge pump circuit. The loop filter has a first capacitive element connected to a charge/discharge node, and a second capacitive element connected to the charge/discharge node through a resistive element. The first capacitive element is charged and discharged by the one charge pump circuit, while the second capacitive element is charged and discharged by the other charge pump circuit. A charging current source of the one charge pump circuit operates simultaneously with a discharging current source of the other charge pump circuit, i.e., the charge pump circuits operate in opposite phase.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. Ser. No.11/241,995, filed Oct. 4, 2005, which is a continuation application ofU.S. Ser. No. 10/739,117, filed Dec. 19, 2003 (now U.S. Pat. No.7,015,735.

The application U.S. Ser. No. 10/739,117, filed Dec. 19, 2003 relates tosubject matter described in a co-pending application Ser. No.10/253,922, filed Sep. 25, 2002 (now U.S. Pat. No. 6,906,596) assignedto the assignee of the present application. The disclosure of theco-pending application is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a phase locked loop (PLL) including avoltage controlled oscillator (VCO) circuit, and more particularly totechniques which are effectively applied to incorporation of a loopfilter on a PLL into a semiconductor chip. More specifically, thepresent invention relates to techniques which are effectively utilizedfor a transmission PLL loop in a high frequency semiconductor circuit,for example, used in radio communication devices such as a portabletelephone for modulating and upconverting a transmission signal.

A radio communication device (mobile communication device) representedby a portable telephone typically comprises a semiconductor integratedcircuit (generally referred to as a high frequency IC) which hasfunctions of upconverting and modulating a transmission signal,downconverting and demodulating a received signal, and the like; asemiconductor integrated circuit (baseband IC) which has functions ofconverting a transmission signal to I, Q signals, and recoveringreception data from demodulated I, Q signals; an electronic partreferred to as a power module which contains a high frequency poweramplifier, an associated bias circuit, an impedance matching circuit,and the like; and an electronic part referred to as a front end modulewhich contains a transmission/reception switching circuit, a low passfilter, an impedance matching circuit and the like.

In radio communication devices, with the recent trend of reducing thenumber of parts for a smaller size and a lower cost of the devices,conscious efforts have been made to incorporate as many circuits aspossible into a single or several semiconductor integrated circuits. Oneof such efforts includes an attempt to provide a semiconductor chip witha built-in loop filter disposed on a loop of a transmission PLL within ahigh frequency IC.

Generally, a second-order filter FLT as illustrated in FIG. 1 has beenused for the loop filter for a transmission PLL within a high frequencyIC for providing a loop band characteristic required thereto. A chargepump is generally indicated by CP. FIG. 2 is a graph showing thefrequency characteristic of the filter FLT in FIG. 1, which has zero atfrequency f1 and a pole at frequency f2. The frequency f1 and thefrequency f2 in the illustrated loop filter are expressed by thefollowing equations:f1=1/{2πC1R}f2=(C1+C2)/2πC1C2R  (1)

Such a second-order filter employs two capacitive elements, wherein aloop filter for a transmission PLL handles relatively high frequencies,causing the larger capacitive element C1 to have a high capacitive valueon the order of nF (nanofarad), which makes it difficult to integratethe capacitive element C1 itself on a chip. Thus, an external element isoften used for the capacitive element C1 to make up the loop filter.

On the other hand, in order to reduce the capacitance values ofcapacitive elements used in a second-order loop filter, a technique hasbeen developed and proposed for providing each capacitive element with acharge pump disposed in front thereof for charging and discharging thecapacitive element, for example, as illustrated in FIG. 3 (see, forexample, 0018-9200/02 IEEE “A Fully Integrated CMOS FrequencySynthesizer With Charge-Averaging Charge Pump and Dual-Path Loop Filterfor PCS- and Cellular-CDMA Wireless System”).

SUMMARY OF THE INVENTION

The loop filter illustrated in FIG. 3 comprises a charge pump CP1 madeup of a charging current source I11 (=i1) and a discharging currentsource I12 (=i1) for establishing a frequency characteristic having acutoff frequency f2, as indicated by a one-dot chain line A in FIG. 5,together with a capacitor C2 and a resistor R; and a charge pump CP2made up of a charging current source I21 (=i2) and a discharging currentsource I22 (=i2) for establishing a frequency characteristic, asindicated by a broken line B in FIG. 5, together with a capacitor C1.Then, these frequency characteristics are synthesized by an adder ADD torealize a frequency characteristic, as indicated by a solid line C inFIG. 5, which has a zero point at f1 near the intersection of the curvesA, B, and a pole at f2.

It should be noted that when the charge-pumps CP1, CP2 are notseparated, a current flows from the current source I11 to I22 or fromI21 to I12, so that the adder ADD in FIG. 3 may be actually representedas shown in FIG. 4. BFF in FIG. 4 represents a buffer such as an emitterfollower. The zero-point frequency f1 and pole frequency f2 in this loopfilter are expressed by the following equations:f1=1/{2πC1(i1/i2)R}f2=1/(2πC2R)  (2)

As can be understood from a comparison of Equation (1) with Equation(2), the frequencies f1 and f2 are determined by the ratio of thecapacitors C1 and C2 in the loop filter of FIG. 1, whereas thefrequencies f1 and f2 are determined by the ratio of the currents i1 andi2 in the loop filter of FIG. 3. Therefore, when the current i2 isreduced, for example, to one tenth of i1, the result is equivalent tothe ratio of the capacitors C1, C2 equal to or higher than 10:1 in theloop filter of FIG. 1, even if the capacitors C1, C2 have similarcapacitance values. More specifically, the capacitor C1 in the loopfilter of FIG. 1 has a capacitance value on the order of nF, whereas thecapacitance values of the capacitors C1, C2 in the loop filter of FIG. 3can be reduced to several hundred pF (picofarad) which is one order ofmagnitude smaller. As a result, the loop filter can be integrated on achip.

However, in an actual circuit implemented as illustrated in FIG. 4,noise generated by the buffer BFF causes an increase in phase error of aPLL loop. Furthermore, when the buffer BFF is applied to a transmissionPLL, noises generated from the buffer BFF under operation intrudes intothe supply voltage and causes a problem of increasing side-bandspurious.

Also, in the charge pumps CP1, CP2, fluctuations in the supply voltagemay cause a change in the current values i1, i2 of the current sources,depending on the type of circuit, resulting in a deviation of the loopgain from a desired value. To avoid this deviation, it is contemplatedto provide a voltage regulator through which the charge pumps areprovided with the supply voltage. However, although a voltage regulatoris typically provided with a band gap reference circuit for preventingthe generated voltage from varying due to the fluctuating supply voltageand a change in temperature, the band gap reference circuit can be asource of noise in a radio communication system to increase a phaseerror of the PLL loop and side-band spurious.

Further, since variation of the supply voltage also cause a voltagecontrolled oscillator (VCO) circuit to vary an oscillation frequency,the VCO is desirably supplied with a supply voltage through a voltageregulator. However, in doing so, similar problems arise as is the casein the charge pumps, i.e., the band gap reference circuit within thevoltage regulator acts as a noise source to increase the phase error ofthe PLL loop and side-band spurious.

It is an object of the present invention to provide a loop filter whichcomprises a second- or higher order filter including capacitiveelements, the capacitance ratio of which can be reduced, allowing theintegration of the loop filter on a chip.

It is another object of the present invention to provide a PLL circuitwhich is free from an increase in phase error even if a loop filter isintegrated on a chip.

It is a further object of the present invention to provide a PLL circuitwhich is free from an increase in side-band spurious even if a loopfilter is integrated on a chip when the PLL circuit is applied for usein a transmission PLL.

It is a further object of the present invention to provide a voltageregulator for supplying a supply voltage to charge pumps and atransmission VCO while protecting the charge pumps and transmission VCOfrom adverse effects due to noise generated by a band gap referencecircuit.

Representative features of the present invention herein disclosed willbe generally described as follows.

Specifically, in a first aspect of the present invention, asemiconductor integrated circuit has a built-in PLL circuit whichincludes two charge pump circuits for charging and dischargingcapacitive elements of a loop filter in response to signals generated bya phase comparator circuit. One of the two charge pump circuits hascurrent sources which generate current values smaller than thosegenerated by current sources of the other charge pump circuit. The loopfilter has a first capacitive element connected to a charge/dischargenode, and a second capacitive element connected to the charge/dischargenode through a resistive element. The first capacitive element ischarged and discharged by the one charge pump circuit, while the secondcapacitive element is charged and discharged by the other charge pumpcircuit. A charging current source of the one charge pump circuitoperates simultaneously with a discharging current source of the othercharge pump circuit. In other words, the two charge pump circuits areconfigured to operate in opposite phase.

According to the semiconductor integrated circuit described above, twocapacitive elements having smaller capacitance values can be employed byreducing the ratio of the current values supplied from the currentsources which form part of the one charge pump circuit to the currentvalues supplied from the current sources which form part of the othercharge pump circuit. The reduction in the capacitance values of thecapacitive elements permits the loop filter to be integrated on thesemiconductor IC chip. In addition, since the one charge pump circuitoperates in opposite phase to the other charge pump circuit, a need iseliminated for disposing a buffer, which can be a noise source, betweenthe resistive element and second capacitive element, thereby making itpossible to prevent an increased phase error of the PLL loop andincreased side-band spurious.

In a second aspect of the present invention, a semiconductor integratedcircuit having a built-in PLL circuit includes a voltage controlledoscillator circuit which operates with a supply voltage generated by avoltage regulator that relies on a base-emitter voltage of a bipolartransistor to generate a predetermined voltage with less source voltagedependency. The voltage regulator includes a voltage regulating circuitwhich utilizes the base-emitter voltage of the bipolar transistor as areference voltage for generating the predetermined voltage. The voltageregulator also includes a first resistive element connected in serieswith the bipolar transistor for generating the base-emitter voltage, aseries connection of a second resistive element and a switching elementconnected in parallel with the first resistive element, and atemperature detector circuit. The switching element is controlled by anoutput signal of the temperature detector circuit to switch the voltagegenerated by the voltage regulator in accordance with the temperaturedetected by the temperature detector circuit.

According to the Semiconductor Integrated circuit in the second aspectdescribed above, since the voltage regulator relies on the base-emittervoltage of the bipolar transistor to generate the predetermined voltage,the voltage regulator eliminates a reference voltage generator circuitwhich can be a noise source. Thus, the resulting voltage regulator willnot adversely affect the charge pumps and transmission VCO with noisewhich would be otherwise generated by the reference voltage generatorcircuit. Also, since the base-emitter voltage has a negative temperaturecharacteristic, the voltage generated by the voltage regulator hastemperature dependency. However, the switching element is controlled bythe output signal from the temperature detector circuit to switch thegenerated voltage in accordance with the detected temperature, so thatthe charge pump and transmission VCO can be supplied with a voltage withless temperature dependency.

Advantages provided by representative aspects of the invention disclosedherein may be summarized as follows.

Specifically, the loop filter can be integrated on a chip. One chargepump circuit is operated in opposite phase to the other charge pumpcircuit to eliminate the need for a buffer, which can be a noise source,disposed between the resistive element and second capacitive element,thereby making it possible to prevent an increased phase error of thePLL loop and increased side-band spurious.

Also, the voltage regulator, which does not include a reference voltagegenerator circuit, will not adversely affect the charge pump ortransmission VCO.

Even if the voltage generated by the voltage regulator has temperaturedependency, the switching element is controlled by the output signalfrom the temperature detector circuit to switch the generated voltage inaccordance with the detected temperature, making it possible to supplythe charge pumps and transmission VCO with a voltage with lesstemperature dependency.

The above and other objects, novel features, and advantages of thepresent invention will become apparent from the following description ofthe specification with reference to the accompanying drawings.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram illustrating an exemplary configuration of asecond-order loop filter provided on a conventional PLL loop;

FIG. 2 is a characteristic graph showing the frequency characteristic ofthe loop filter illustrated in FIG. 1;

FIG. 3 is a circuit diagram illustrating another exemplary configurationof a conventional second-order loop filter;

FIG. 4 is a circuit diagram illustrating the configuration of an actualloop filter in FIG. 3;

FIG. 5 is a characteristic graph showing the frequency characteristic ofthe loop filter illustrated in FIG. 3;

FIG. 6 is a circuit diagram illustrating a second-order loop filteraccording to one embodiment of the present invention;

FIG. 7 is a circuit diagram illustrating an exemplary configuration of aloop filter according to another embodiment of the present invention,and a PLL loop which employs this loop filter;

FIG. 8 is a circuit diagram illustrating an exemplary configuration of aloop filter according to a further embodiment of the present invention,and a PLL loop which employs this loop filter;

FIG. 9 is a circuit diagram illustrating another exemplary configurationof a PLL loop which employs a loop filter according to one embodiment ofthe present invention;

FIG. 10 is a circuit diagram illustrating one embodiment of a voltageregulator for supplying a supply voltage to a transmission oscillatorcircuit (TXVCO);

FIG. 11 is a temperature characteristic graph showing a voltageoutputted from the voltage regulator in the embodiment of FIG. 10, andthe temperature dependency of the potential at an internal node within atemperature detector circuit;

FIG. 12 is a temperature characteristic graph showing the temperaturedependency of the output voltage in an exemplary modification to thevoltage regulator in FIG. 10;

FIG. 13 is a circuit diagram illustrating another exemplaryconfiguration of a temperature detector of the voltage regulator in theembodiment of FIG. 10;

FIG. 14 is a circuit diagram illustrating one embodiment of a phasecomparator circuit which operates with a voltage applied from thevoltage regulator in the foregoing embodiment; and

FIG. 15 is a block diagram illustrating an exemplary configuration of aradio communication system including a high frequency IC (RF-IC) whichemploys the PLL comprising the loop filter and phase comparator circuitin the foregoing embodiments for a transmission PLL.

DESCRIPTION OF THE INVENTION

In the following, preferred embodiments of the present invention will dedescribed with reference to the accompanying drawings.

First Embodiment

FIG. 6 illustrates one embodiment of a loop filter which can be providedon a PLL loop. The loop filter in this embodiment comprises a firstcharge pump CP1 which is made up of regulated current sources I11, I12,and switches SW11, SW12 connected in series with the regulated currentsources I11, I12, respectively; a capacitor C2 connected between anoutput node N1 of the charge pump CP1 and a ground point; a resistor Rand a capacitor C1 connected between the output node N1 and ground pointin parallel with the capacitor C2; and a second charge pump CP2 which ismade up of regulated current sources 121, 122, and switches SW21, SW22connected in series with the regulated current sources I21, I22,respectively, and has an output node N2 connected to a connection nodeN3 at which the resistor R is connected to the capacitor C1.

In the loop filter (FIG. 6) of this embodiment, the switch SW21 forcharging the second charge pump CP2 is controlled on/off by a downsignal DOWN from a previous circuit (phase comparator circuit) inopposite phase to the switch SW11 for charging the first charge pumpCP1. The switch SW22 for discharging the second charge pump CP2 iscontrolled on/off by an up signal UP from the previous circuit inopposite phase to the switch SW12 for discharging the first charge pumpCP1.

When used herein, the foregoing operation of the second charge pump CP2is referred to as the “opposite-phase operation.” In this embodiment,the second charge pump CP2 operated in opposite phase eliminates theneed for a buffer which is disposed between the resistor R andconnection node N3 in the loop filter illustrated in FIG. 4. It shouldbe noted that in the loop filter of FIG. 4, the switch SW21 associatedwith the second charge pump CP2 is controlled on/off by a signal inphase with a signal applied to the switch SW11 associated with the firstcharge pump CP1, and the switch SW22 is controlled on/off by a signal inphase with a signal applied to the switch SW12.

The output voltage Vout of the loop filter in FIG. 6 has the samefrequency characteristic as that shown in FIG. 2, where a zero point ispresent at f1, and a pole at f2. The zero point frequency f1 and polefrequency f2 are expressed by the following equation $\begin{matrix}{{(3)\text{:}}{f_{1} = \frac{1}{2\quad{\pi \cdot \frac{C_{1}}{1 - \beta} \cdot R}}}{f_{2} = {\frac{C_{1} + C_{2}}{2{\pi\quad \cdot C_{1} \cdot C_{2} \cdot R}} \approx \frac{1}{2\quad{\pi \cdot C}\quad{1 \cdot R}}}}{\beta = \frac{i\quad 2}{i\quad 1}}} & \quad\end{matrix}$

It can be understood from the equation (3) above that when the ratio β(=i2/i1) of the current i2 of the current source associated with thesecond charge pump CP2 to the current i1 of the current sourceassociated with the first charge pump CP1 is set to 0.9, the resultingfilter can have the frequencies f1, f2 as if it had the capacitor C1,the capacitance value of which is reduced by a factor of ten.

It is therefore possible to reduce the capacitance value of thecapacitor C1 to a similar capacitance value to the capacitor C2. In atransmission PLL contained in a high frequency IC intended by theinventors for application, since C2 has several hundred picofarads, C1can be reduced as well to approximately several hundred picofarads.Then, since capacitors having capacitance values as low as the foregoingcan be sufficiently formed on a semiconductor chip, the loop filter canbe integrated on the semiconductor chip. Further, since this embodimenteliminates the need for the buffer which would be required in the loopfilter of FIG. 4 that controls the switches in phase, it is possible toprevent noise generated in the buffer from introducing into the outputVout to increase a phase error of the PLL loop. Moreover, theelimination of the buffer can result in a corresponding reduction in thecircuit size.

Next, other embodiments of the loop filter according to the presentinvention will be described with reference to FIGS. 7 and 8. Theseembodiments permit the PLL to rapidly pull in upon start of a voltagecontrolled oscillator (VCO) to enable a high speed lockup operation. Aphase comparator, designated by PD, compares the phase of an oscillatingsignal from the VCO or a signal resulting from a frequency division ofthe oscillating signal with the phase of a reference signal φref togenerate on/off control signals UP, DOWN for the current sources of thecharge pump CP1, CP2.

The embodiment illustrated in FIG. 7 comprises a series connection of aresistor R1 and a switch SW1 in parallel with the resistor R in theembodiment of FIG. 6. The switch SW1 is temporarily turned on by acontrol signal upon start of the VCO.

Though not apparent in FIG. 7, the current sources associated with thecharge pumps CP1, CP2 are also controlled by the control signals toincrease their current values. The switch SW1 is turned off after thePLL is locked up, and the current values of the current sourcesassociated with the charge pumps CP1, CP2 are also returned to normalvalues. For scaling up/down the current values of the current sourcesassociated with the charge pumps CP1, CP2, an auxiliary current sourceand switch, for example, may be provided in parallel with a referencecurrent source, such that the switch is turned on/off to change thecurrent value.

By temporarily turning on the switch SW1 within the loop filter uponstart of the VCO in the foregoing manner, the capacitors C1, C2 can berapidly charged to speed up the pull-in of the PLL. It should be notedthat the switch SW1 alone could be disposed in parallel with theresistor R only for purposes of speeding up the pull-in of the PLL, butthis may reduce a phase margin to result in oscillations. For thisreason, the resistor R1 is additionally disposed in series with theswitch SW1 to ensure a sufficient phase margin. In addition, a pluralityof sets of the switch SW1 and resistor RF may be provided in parallelwith the resistor R to correct a discrepancy in the characteristic ofthe filter due to variations in the course of manufacturing and thelike.

On the other hand, the embodiment in FIG. 8 additionally provides aswitch SW1 in parallel with the resistor R, and a switch SW2 and aregulated voltage source VS1 in parallel with the capacitor C1 in theembodiment of FIG. 6. These switches SW1, SW2 are temporarily turned onbefore the VCO is started, and turned off after the PLL is locked up bythe associated control signals. A voltage selected for the regulatedvoltage source VS1 is close to a VCO control voltage which correspondsto a frequency at which the PLL is locked.

In the embodiment of FIG. 8, since the VCO is started with a controlvoltage terminal of the VCO applied with a voltage which is supposed tobe applied when the PLL is locked, the PLL can be more rapidly pulled inthan when the voltage applied to the control voltage terminal isgradually increased from zero volt.

FIG. 9 illustrates an exemplary modification to the PLL which employsthe loop filter of the foregoing embodiment.

In the PLL in the embodiments of FIGS. 7 and 8, the charge pumps CP1,CP2 for charging and discharging the capacitors of the loop filter iscontrolled by the signals from the single phase comparator PD. In such aconfiguration, since the distance from the phase comparator PD to thecharge pump CP1 differs from the distance from the phase comparator PDto the charge pump CP2, there is a difference between signal delay timesof the two parts, which causes a discrepancy between the switchingtiming of the charge pump CP1 and the switching timing of the chargepump CP2, possibly resulting in unwanted spurious.

To address this problem, in the embodiment of FIG. 9, a phase comparatorPD2 is provided for controlling the charge pump CP2 separately from thephase comparator PD1 for controlling the charge pump CP1, so that thesignal delay amounts can be individually adjusted. In this way, theswitching timing of the charge pump CP1 can be matched with theswitching timing of the charge pump CP2 to prevent the unwantedspurious.

Second Embodiment

Generally, the voltage controlled oscillator (VCO) suffers from avarying oscillation frequency as the supply voltage fluctuates. Toprevent the varying oscillation frequency, the present inventionsupplies a transmission oscillator circuit (TXVCO) with a supply voltagefrom a voltage regulator. FIG. 10 illustrates one embodiment of avoltage regulator for supplying a transmission oscillator circuit(TXVCO) with a supply voltage.

The voltage regulator illustrated in FIG. 10 comprises a regulator unit110 for generating a desired supply voltage using a base-emitter voltageVbe of a bipolar transistor; and a temperature detector unit 120 forgenerating a control signal to the regulator unit 110. Conventionalvoltage regulators typically have a band gap reference circuit. The useof the band gap reference circuit permits the voltage regulator togenerate a stable supply voltage without source voltage dependencyregardless of changes in temperature.

However, in a particular voltage regulator for generating a supplyvoltage which is supplied to a charge pump that forms part of atransmission oscillator circuit or a transmission PLL, noise generatedin the band gap reference circuit is transferred to the transmissionoscillator circuit or charge pump through the generated voltage, causinga lower CN ratio.

In the voltage regulator of the embodiment illustrated in FIG. 10, sincethe regulator unit 110 does not include a band gap reference circuit, nonoise will be transferred to the transmission oscillator circuit orcharge pumps through a generated voltage. While the temperature detectorunit 120 is provided with a voltage regulator circuit which comprises aband gap reference circuit, the temperature detector unit 120 utilizesthe band gap reference circuit for generating the control signal whichturns on/off a MOS switch within the regulator unit 110 to switch agenerated voltage, so that noise generated by the band gap referencecircuit will not be transferred to the transmission oscillator circuitor charge pumps.

Consequently, in a PLL circuit having a transmission oscillator circuitand charge pumps which is operated with a voltage generated by thevoltage regulator, it is possible to prevent an increased phase errorand increased side-band spurious.

The regulator unit 110 is made up of a voltage regulator circuit 111which comprises MOS transistors M1, M2 having their source terminalsconnected to a supply voltage terminal Vcc and their gates coupled toeach other to form a current mirror circuit; a bipolar transistor Q1connected in series with the MOS transistor M1; a bipolar transistor Q2connected in series with the MOS transistor M2; and resistors R1, R2,and a voltage converter circuit 112 using a differential amplifier.

In the voltage regulator circuit 111, the transistor Q1 has a collectorconnected to a base of the transistor Q2, and a base applied with avoltage divided by the resistors R1, R2 connected in series with thetransistor Q2. Thus, the voltage regulator circuit 111 generates aregulated voltage Va which is determined by the base-emitter voltage Vbeof the transistor Q1 and the ratio of the resistors R1, R2. In addition,a series connection of a switch MOS transistor M3 and a resistor R3 isconnected in parallel with the resistor R1, such that the generatedvoltage Va is switched in two steps in accordance with an on- oroff-state of the switch MOS transistor M3.

The voltage converter circuit 112 comprises a pair of differential inputtransistors Q3, Q4; a resistor R4 connected between a common emitter ofthe transistors Q3, Q4 and a ground point; active load MOS transistorsM4, M5 connected to collectors of the transistors Q3, Q4, respectively;an output transistor M6 having a gate terminal connected to thecollector of one differential input transistor Q3; and a transistor Q5and a resistor R6 connected in series between the supply voltage Vcc andground point. The transistor Q5 is applied with an output voltage Vregat the base terminal, while the differential input transistor Q3 isapplied with the regulated voltage Va generated by the voltage regulatorcircuit 111 at the base terminal, and an emitter voltage of thetransistor Q5 is fed back to the base terminal of the other differentialinput transistor Q4, so that the voltage converter circuit 112 operatesin such a manner that the emitter voltage Vb of the transistor Q5matches the regulated voltage Va from the voltage regulator circuit 111.

The voltages Va, Vb, Vreg generated by the appropriate components in thevoltage regulator of FIG. 10 are expressed as follows:Va = {(r  1 + r  2)/r  2} × VbeQ  1 Vb = Va − VbeQ  3 + VbeQ  4 ≈ Va$\begin{matrix}{{Vreg} = {{Vb} + {{VbeQ}\quad 5}}} \\{= {{Va} + {{VbeQ}\quad 5}}} \\{= {{\{ {{( {{r\quad 1} + {r\quad 2}} )/r}\quad 2} \} \times {VbeQ}\quad 1} + {{VbeQ}\quad 5}}}\end{matrix}$where r1, r2 represent the resistance values of the resistors R1, R2;and VbeQ1, VbeQ3, VbeQ4, VbeQ5 represent the base-emitter voltages ofthe bipolar transistors Q1, Q3, Q4, Q5, respectively.

Further, when VbeQ1=VbeQ5 stands, the foregoing equation can betransformed as follows supposing that VbeQ1=VbeQ5=Vbe stands:Vreg={(r1+2r2)/r2}×Vbe

It can be understood from this equation that the voltage regulator ofFIG. 10 can generate a voltage which is determined by the base-emittervoltage Vbe of the bipolar transistor and the resistance ratio of theresistors R1, R2. As is well known, the base-emitter voltage Vbe of thebipolar transistor has a negative temperature characteristic. Therefore,the output voltage Vreg generated by the voltage regulator of FIG. 10would fluctuate following a change in temperature unless appropriateactions were taken therefor (Vreg is lower as the temperature ishigher).

In the embodiment of FIG. 10, to address fluctuations in the outputvoltage Vreg, a series connection of the switch MOS transistor M3 andresistor R3 is connected in parallel with the resistor R1 of the voltageregulator circuit 111, such that the generated voltage Va is switched intwo steps in accordance with an on- or off-state of the switch MOStransistor M3.

In response to the voltage Va switched in two steps, the voltageregulator also switches the output voltage Vreg in steps in accordancewith the on- or off-state of the switch MOS transistor M3.

Specifically, as the switch MOS transistor M3 is turned on, which isequivalent to a reduction in the resistance value r1 of the resistor R1in the aforementioned equation which expresses the output voltage Vreg,the voltage Va is reduced so that the voltage regulator generates alower output voltage Vreg. Therefore, the switch MOS transistor M3 isturned on at lower than a proper temperature Tc to reduce the outputvoltage Vreg, while the switch MOS transistor M3 is turned off at theproper temperature Tc or higher to increase the output voltage Vreg.

In this way, the output voltage Vreg of the voltage regulator can becontrolled to decrease as the temperature becomes higher, once rise upat the temperature Tc at one step, and again gradually decrease, asindicated by a solid line in FIG. 11. As a result, by changing theoutput voltage Vreg through the switching of the switch MOS transistorM3, it is possible to reduce the amount of fluctuations in the outputvoltage Vreg of the voltage regulator with respect to the temperature.If the switch MOS transistor M3 is not switched, the output voltage Vregof the voltage regulator will be largely reduced as the temperaturerises, as indicated by a broken line in FIG. 11.

The temperature detector unit 120 in the voltage regulator of FIG. 10 isprovided for generating a signal for switching the switch MOS transistorM3.

The temperature detector unit 120 in this embodiment comprises a biascircuit 121 composed of MOS transistors M7-M9; a differential circuit122 composed of bipolar transistors Q6, Q7, which differ in the emittersize from each other, current mirror MOS transistors M10, M11 forsupplying collector currents to the bipolar transistors Q6, Q7,respectively, and resistors R6, R7 connected between emitters of thebipolar transistors Q6, Q7 and a ground point; an emitter followercircuit 123 a composed of a bipolar transistor Q8 for applying a currentin accordance with a collector voltage of the bipolar transistor Q7, anda resistor R8; a source follower circuit 123 b composed of a MOStransistor M12 for applying a current in accordance with a collectorvoltage of the bipolar transistor Q6, and a resistor R9; and a voltagecomparator circuit 124 composed of bipolar transistors Q9, Q10 which areapplied with a drain voltage of the MOS transistor M12 and an emittervoltage of the bipolar transistor Q8, respectively, at their bases, andhave their emitters coupled to each other to perform a differentialoperation, current mirror MOS transistors M13-M17 which are connected tocollectors of the bipolar transistors Q9, Q10, and MOS transistors M17,M18 which are connected in series with the MOS transistors M13, M17 andhave their gate terminals connected to each other to form a currentmirror; and an inverter circuit 125 which has a hysteresischaracteristic. The transistors Q6, Q7 in the temperature detectorcircuit 122 are set such that the former has an emitter size eight timeslarger than the latter.

In the temperature detector circuit 122 in FIG. 10, the followingequations are derived:I1=(VbeQ7−VbeQ6)/r6Vc=(I1+I2)r7+VbeQ7Vd=I3r9where I1, I2 represent collector currents flowing into the bipolartransistors Q6, Q7, respectively; Vc represents a voltage at the emitterof the bipolar transistor Q8; Vd represents a drain voltage of the MOStransistor M12; 13 represents a current flowing through the resistor R9;and r6, r7, r9 are resistance values of the resistors R6, R7, R9,respectively.

Assuming herein that the MOS transistors MNb, M11, M12 connected to forma current mirror have the same size, I1=I2=I3 is established. In thisevent, paying attention to the bipolar transistors Q6, Q7,VbeQ7=VbeQ6+I1 r6 is established. From this equation, the correctorcurrent I1 of the bipolar transistor Q6 is expressed byI1=(VbeQ7−VbeQ6)/r6. Therefore, the voltages Vc, Vd are transformed asexpressed by the following equations: $\begin{matrix}\begin{matrix}{{Vc} = {{2\quad I\quad 1\quad r\quad 7} + {{VbeQ}\quad 7}}} \\{= {\{ {2( {{{VbeQ}\quad 7} - {{VbeQ}\quad 6}} )r\quad{7/r}\quad 6} \} + {{VbeQ}\quad 7}}}\end{matrix} & {{Equation}\quad 3.1}\end{matrix}$  Vd=(VbeQ7−VbeQ6)r9/r6  Equation 3.2

The base-emitter voltages VbeQ6, VbeQ7 of the bipolar transistors Q6, Q7have negative temperature characteristics. Moreover, in this embodiment,the bipolar transistors Q6, Q7 are set to have the emitter sizes in theratio of 8:1. Therefore, the absolute values of their base-emittervoltages are in a relationship expressed by |VbeQ6|<|VbeQ7|. From thisrelationship, (VbeQ7−VbeQ6) in the foregoing Equation 3.2 is positive.In conclusion, a temperature coefficient of the voltage Vc can bebrought closer to zero if the resistance values r6, r7 of the resistorsR6, R7 are set such that the absolute value of a temperature coefficientin the first term is equal to the absolute value of a temperaturecoefficient in the second term.

On the other hand, it can be understood from Equation 3.2 that atemperature coefficient of the voltage Vd can be adjusted by changing aresistance ratio r9/r6 of the resistors R6 and R9.

In the temperature detector circuit in this embodiment, the resistancevalues r6, r7 are appropriately set for the resistors R6, R7 to bringthe temperature coefficient of the voltage Vc closer to zero, and theresistance ratio r9/r6 is appropriately adjusted for the resistors R6,R9 such that the voltages Vc, Vd intersect near 40° C., as shown in FIG.11.

The voltage comparator circuit 124 compares the voltages Vd, Vc, andchanges its output voltage if Vd is higher than Vc, causing the outputof the inverter 125 to transition to a high level which turns off theswitch MOS transistor M3 in the regulator unit 110. The resistance ratior9/r6 can be changed for the resistors R6, R9 to arbitrarily set atemperature at which the switch MOS transistor M3 switches from on-stateto off-state.

It should be noted that the temperature detector unit 120 in thisembodiment can suffer from noise generated in the temperature detectorcircuit 122, but the noise, if any, will be blocked by the inverter 125,and therefore will never be transferred to the voltage regulator unit110. It is therefore possible to prevent a degradation in thecharacteristic of a circuit which operates with the voltage generated bythe voltage regulator. In addition, the voltage comparator circuit 124may be provided with a hysteresis characteristic to prevent the switchMOS transistor M3 from repeatedly turning on and off due to fluctuationsin temperature.

The foregoing embodiment has been described in connection with a circuitwhich is configured to switch the voltage generated by the voltageregulator in two steps in accordance with the temperature.

Alternatively, the regulator unit 110 in FIG. 10 may comprise aplurality of sets of the switch MOS transistor M3 and resistor R3, andthe temperature detector unit 120 may comprise a plurality of sets ofthe source follower circuit 123 b composed of the MOS transistor M12 andresistor R9, and the comparator circuit 124 composed of a pair of thedifferential transistors Q9, Q10, to switch the voltage generated by thevoltage regulator in multiple steps, for example, as illustrated in FIG.12, in accordance with the temperature.

FIG. 13 illustrates an exemplary modification to the voltage regulatoraccording to the second embodiment. This modified voltage regulatorcomprises normal-on switching elements SW1, SW2, SW3 connected betweenthe source terminal of the MOS transistor M8 and the supply voltageterminal Vcc, between the gate and drain of the MOS transistor M10, andbetween the gate terminals of the MOS transistors M10 and M12, andnormal-off switching elements SW4, SW5 connected between the gateterminals of the MOS transistors M10, M11 and the supply voltageterminal Vcc, and between the gate terminal of the MOS transistor M12and the supply voltage terminal Vcc, respectively, in the temperaturedetector unit 120. With the provision of these switching elementsSW1-SW5, the temperature detector circuit 120 is operated only whentemperature information is required, such as upon start of the PLLcircuit, and is otherwise made inoperative to save the powerconsumption. The normal-off switching elements SW4, SW5 are provided forpreventing a through current from flowing through the circuit due to afloating potential state at an internal node in the circuit during aninoperative state.

Also, in this modified voltage regulator, the inverter circuit 125 inthe embodiment of FIG. 10 is replaced with a flip-flop 126 for latchingthe output of the temperature detector circuit 120. This flip-flop 126latches the output of the temperature detector circuit 120 immediatelybefore the VCO is started, so that the VCO can be obviated from instableoperations caused by variations in the output of the temperaturedetector circuit 120 after the start of the VCO due to fluctuations intemperature.

Third Embodiment

FIG. 14 illustrates one embodiment of a phase comparator circuit whichis applied with the voltage from the voltage regulator in the foregoingembodiment for operations. The phase comparator circuit 236 in thisembodiment comprises a signal comparator unit (left-hand side circuit)361 and an output unit (right-hand side circuit) 362. As illustrated inFIG. 14, the signal comparator unit 361 comprises a cascadedconfiguration of differential circuits, wherein a current regulatingtransistor Q30 is connected to emitters of a lower differentialtransistor pair Q31, Q32, and differential transistor pairs Q33, Q34 andQ35, Q36 are connected to collectors of the lower differentialtransistor pair Q31, Q32. The transistors Q33, Q35 have their collectorscoupled to each other, which are then connected to a common loadtransistor Q37. Likewise, the transistors Q34, Q36 also have theircollectors coupled to each other, which are connected to a common loadtransistor Q38.

In this embodiment, differential reference signals φref, /φref areapplied to base terminals of the lower differential transistor pair Q31,Q32, respectively, while a signal φTX from the VCO is applied to baseterminals of the transistors Q33, Q36, and a signal /φTX having thephase opposite to that of the signal φX is applied to base terminals ofthe transistors Q34, Q35 in the upper differential transistor pairsQ33-Q36, so that the transistors Q33-Q36 generate voltages at theircollectors in accordance with a phase difference between φref and φTX.

The output unit 362 comprises transistors Q41, Q44 connected in acurrent mirror configuration with the load transistor Q37 in the signalcomparator unit; transistors Q42, Q43 connected in a current mirrorconfiguration with the load transistor Q38; transistors Q45-Q48connected in series with the transistors Q41-44, respectively; andtransistors Q49, Q50 for applying bias voltages to base terminals of thetransistors Q45, Q46 and Q47, Q48, respectively. The transistors Q45,Q46 have their bases connected in common, the transistors Q47, Q48 alsohave their bases connected in common, the transistors Q49, Q50 havetheir collectors connected to a supply voltage terminal, and their basesconnected to collectors of Q41, Q43, respectively, such that biasvoltages are applied from an emitter of the transistor Q49 to the commonbase of the transistors Q45, Q46, and from an emitter of the transistorQ50 to the common base of the transistors Q47, Q48, respectively.

In the phase comparator circuit of this embodiment, the transistors Q41,Q44 and transistors Q42, Q43 are respectively set to have their emittersizes in a ratio of 10:9. The transistors Q45, Q46 have the same emittersize, and the transistors Q47, Q48 also have the same emitter size.Consequently, the transistors Q44, Q48 are applied with 9/10 of acurrent applied to the transistors Q42, Q46.

Further, the transistor Q48 is applied at its base with a signal inphase with a signal applied to a base of the transistor Q42, while thetransistor Q44 is applied at its base with a signal in phase with asignal applied to a base of the transistor Q41, so that the pairs oftransistors Q42, Q48 and Q46, Q44 are controlled in phase, respectively.Thus, the transistor Q42 corresponds to the regulated current sourceI11, while the transistor Q46 corresponds to the regulated currentsource I12 in FIG. 6. Also, the transistor Q44 corresponds to theregulated current source I21, while the transistor Q48 corresponds tothe regulated current source 122. The phase comparator circuit of theembodiment in FIG. 14 configured in the foregoing manner operates with asupply voltage which is the voltage Vreg supplied from the regulatorunit 110 with less source voltage dependency and temperature dependency,allowing the regulated current sources to maintain consistent currentvalues against fluctuations in the supply voltage and changes intemperature for charging and discharging a loop filter.

Fourth Embodiment

Referring next to FIG. 15, description will be made on an exemplaryconfiguration of a radio communication system including a high frequencyIC (RF-IC) which employs the PLL comprising the loop filter and phasecomparator circuit according to the foregoing embodiments for atransmission PLL.

As illustrated in FIG. 15, the radio communication system in thisembodiment comprises an antenna 400 for transmitting and receivingsignal radiowaves; a switch 410 for switching between a transmission anda reception mode; bandpass filters 420 a-420 d each comprised of a SAWfilter for removing unwanted waves from a received signal, and the like;a high frequency power amplifier circuit (power module) 430 foramplifying a transmission signal; a high frequency IC 200 fordemodulating a received signal and modulating a transmission signal; anda baseband circuit 300 for converting transmission data into I, Qsignals and controlling the high frequency IC 200. In this embodiment,the high frequency IC 200 and baseband circuit 300 are implemented onseparate semiconductor chips, respectively, as individual semiconductorintegrated circuits.

Though not particularly limited, the high frequency IC 200 in thisembodiment is configured to be capable of modulating and demodulatingsignals in four frequency bands conforming to the followingcommunication schemes: GSM850 and GSM900, DCS1800, and PCS1900.Accordingly, the bandpass filters provided in the radio communicationsystem include the filter 420 a for passing therethrough a receivedsignal in the frequency band of GSM 850; filter 420 b for passingtherethrough a received signal in the frequency band of GSM 900; filter420 c for passing therethrough a received signal in the frequency bandof DCS1800; and filter 420 d for passing therethrough a received signalin the frequency band of PCS1900.

The high frequency IC 200 in this embodiment is generally divided into areception-related circuit RXC; a transmission-related circuit TXC; and acontrol-related circuit which comprises circuits common to thetransmission- and reception-related circuits such as a control circuit,a clock generator circuit, and the like.

The reception-related circuit RXC comprises low noise amplifiers 210a-210 d for amplifying received signals in the respective frequencybands of GSM850, GSM900, DCS1800, and PCS1900, respectively; a frequencydivider/phase shifter circuit 211 for dividing a local oscillatingsignal φRF generated by a high frequency oscillator circuit (RFVCO) 250and generating orthogonal signals which are 90° out of phase from eachother; mixer circuits 212 a, 212 b for mixing received signals amplifiedby the low noise amplifiers 210 a-210 d with the orthogonal signalsgenerated by the frequency divider/phase shifter circuit 211 todemodulate and downconvert an I-signal and a Q-signal; high gainamplifier units 220A, 220B common to the respective frequency bands foramplifying the demodulated I- and Q-signals for delivery to the basebandLSI 300; and an offset cancel circuit 213 for canceling input DC offsetsof amplifiers in the high gain amplifier units 220A, 220B.

The high gain amplifier unit 220A comprises a plurality of low passfilters LPF11, LPF12, LPF13, LPF14 and gain control amplifiers PGA11,PGA12, PGA13 which are alternately connected in series; and an amplifierAMP1 connected at the final stage, and amplifies a demodulated I-signalto a predetermined amplitude level while removing unwanted waves.Likewise, the high gain amplifier unit 220B comprises a plurality of lowpass filters LPF21, LPF22, LPF23, LFP24 and gain control amplifiersPGA21, PGA22, PGA23 which are alternately connected in series; and anamplifier AMP2 connected at the final stage, and amplifies a demodulatedQ-signal to a predetermined amplitude level.

The offset cancel circuit 213 comprises A/D converter circuits (ADC)provided in correspondence to the respective gain control amplifiersPGA11-PGA23 for converting output potential differences of the gaincontrol amplifiers PGA11-PGA23 when their input terminals areshort-circuited; D/A converter circuits (DAC) each for generating aninput offset voltage based on the result of the conversion made by anassociated A/D converter, such that the resulting input offset voltagereduces a DC offset to zero in the output of the corresponding gaincontrol amplifier PGA11-PGS23, and applying the generated input offsetvoltage to a differential input of the corresponding gain controlamplifier; and a control circuit for controlling the A/D convertercircuits (ADC) and D/A converter circuits (DAC) to perform an offsetcancel operation.

The transmission-related circuit TXC comprises an oscillator circuit(IFVCO) 230 for generating an oscillating signal φIF at an intermediatefrequency such as 640 MHz, for example; a phase shifter/frequencydivider circuit 232 for dividing the oscillating signal φIF generated bythe oscillator circuit 230 and generating orthogonal signals which are90° out of phase from each other; modulator circuits 233 a, 233 b eachcomprised of a mixer for modulating the generated orthogonal signal withthe I-signal or Q-signal supplied from the baseband circuit 300; anadder 234 for adding the modulated signals; transmission oscillatorcircuits (TXVCO) 240 a, 240 b each for generating a transmission signalφTX at a predetermined frequency; an offset mixer 235 a for synthesizinga feedback signal which is the transmission signal φTX outputted fromthe transmission oscillator circuit 240 a, 240 b, extracted by a coupleror the like, with a signal φRF′ divided from the oscillating signal φRFgenerated by the high frequency oscillator circuit (RFVCO) 250 togenerate a signal at a frequency corresponding to a difference infrequency between the two signals; a phase comparator circuit 236 forcomparing the output of the offset mixer 235 a with a signal TXIFresulting from the synthesis in the adder 234 to detect a frequencydifference and a phase difference; a charge pump/loop filter 237 forgenerating a voltage in accordance with the output of the phase detectorcircuit 236; and buffer circuits 238 a, 238 b each for convertingdifferential outputs of the associated TXVCO 240 a, 240 b into a singlesignal for delivery to the outside.

One of the transmission oscillator circuits 240 a, 240 b generates asignal in a band from 850 to 900 MHz for GSM, and the other generates asignal in a band from 1800 to 1900 MHz for DCS and PCS.

The high frequency IC chip 200 in this embodiment further comprises acontrol circuit 260 for controlling the overall chip; an RF synthesizer261 which makes up an RF PLL circuit together with the high frequencyoscillator circuit (RFVCO) 250; an IF synthesizer 262 which makes up anIF PLL circuit together with the intermediate frequency oscillatorcircuit (IFVCO) 230; and a reference oscillator circuit (TCXO) 264 forgenerating a clock signal φref which serves as a reference signal forthese synthesizers 261, 262. The synthesizers 261, 262 are each composedof a phase comparator circuit, a charge pump, a loop filter, and thelike.

Since the reference oscillating signal φref is required to be accuratein frequency, an external quartz vibrator is connected to the referenceoscillator circuit 264. The frequency selected for the referenceoscillating signal φref may be 26 MHz or 13 MHz. This is because quartzvibrators at such frequencies are general purpose parts and are readilyavailable on the market.

The control circuit 260 comprises a control register which is set basedon a signal from the baseband IC 300. Specifically, the high frequencyIC 200 is supplied with a clock signal CLK for synchronization, a datasignal SDATA, and a load enable signal LEN as a control signal from thebaseband IC 300. As the load enable signal LEN is asserted to a validlevel, the control circuit 260 sequentially captures the data signalSDATA transmitted thereto from the baseband IC 300 in synchronism withthe clock signal CLK, and sets the captured data signal SDATA into thecontrol register. Though not particularly limited, the data signal SDATAmay be transmitted in series. The baseband IC 300 may be based on amicroprocessor or the like. The data signal SDATA includes a commandissued from the baseband IC 300 to the high frequency IC 200.

In this embodiment, the phase detector circuit 236, charge pump/loopfilter 237, transmission oscillator circuits (TXVCO) 240 a, 240 b, andoffset mixer 235 make up a transmission PLL circuit (TXPLL) forperforming a frequency conversion. In the multi-band type radiocommunication system according to this embodiment, the control circuit260 changes the frequency φRF of the oscillating signal from the highfrequency oscillator circuit 250 in accordance with a particular channelin use, and changes the frequency of a signal supplied to the offsetmixer 235 a in accordance with a GSM mode or a DCS/PCS mode to switchthe transmission frequency, for example, in response to a command fromthe baseband IC 300 during transmission and reception.

On the other hand, different values are set for the oscillationfrequency of the high frequency oscillator circuit (RFVCO) 250 in thereception mode and transmission mode, respectively. In the transmissionmode, the oscillation frequency fRF of the high frequency oscillatorcircuit (RFVCO) 250 is set, for example, in a range of 3,616 to 3,716MHz for GSM850, in a range of 3,840 to 3,980 MHz for GSM900, in a rangeof 3,610 to 3,730 MHz for DCS, or in a range of 3,860 to 3,980 MHz forPCS. Then, the frequency divider circuit divides the oscillationfrequency fRF by four for GSM, or by two for DCS and PCS, and theresulting oscillation frequency is supplied to the offset mixers 235 a,235 b.

The offset mixer 235 a outputs a signal which corresponds to adifference in frequency between the oscillating signal φRF from theRFVCO 250 and the transmission oscillating signal φTX from thetransmission oscillator circuits (TXCVO) 240 a, 240 b (fRF-fTX), and thetransmission PLL (TXPLL) operates such that this difference signalmatches the modulated signal TXIF in frequency. Stated another way, theTXVCOs 240 a, 240 b are controlled to oscillate at a frequency whichcorresponds to the difference in frequency between the oscillatingsignal φRF from the RFVCO 250 (fRF/4 for GSM, and fRF/2 for DCS and PCS)and the modulated signal TXIF.

While the invention created by the inventors has been described inspecific manner with reference to several embodiments thereof, it shouldbe understood that the present invention is not limited to the foregoingembodiments but can be modified in various manners without departingfrom the spirit and scope of the invention. For example, while theforegoing embodiments have been described in connection with asecond-order loop filter to which the present invention is applied, thepresent invention can be applied as well to a third-order loop filter.

In the foregoing description, the present invention made by theinventors has been discussed oscillating signal φRF from the RFVCO 250and the transmission oscillating signal φTX from the transmissionoscillator circuits (TXCVO) 240 a, 240 b (fRF-fTX), and the transmissionPLL (TXPLL) operates such that this difference signal matches themodulated signal TXIF in frequency. Stated another way, the TXVCOs 240a, 240 b are controlled to oscillate at a frequency which corresponds tothe difference in frequency between the oscillating signal φRF from theRFVCO 250 (fRF/4 for GSM, and fRF/2 for DCS and PCS) and the modulatedsignal TXIF.

While the invention created by the inventors has been described inspecific manner with reference to several embodiments thereof, it shouldbe understood that the present invention is not limited to the foregoingembodiments but can be modified in various manners without departingfrom the spirit and scope of the invention. For example, while theforegoing embodiments have been described in connection with asecond-order loop filter to which the present invention is applied, thepresent invention can be applied as well to a third-order loop filter.

In the foregoing description, the present invention made by theinventors has been discussed mainly in connection with a transmissionPLL which forms part of a radio communication system, and a highfrequency IC which contains the transmission PLL, which belong to thefield of utilization that underlies the

1. (canceled)
 2. A radio communication system, comprising: an antennafor transmitting and receiving signal radio waves; a power amplifier foramplifying a transmission signal and outputting an amplifiedtransmission signal to the antenna; a baseband circuit for outputtingsignals to be transmitted; and a semiconductor integrated circuit fordemodulating a reception signal from the antenna and modulating thesignal and generating the transmission signal to the power amplifier,wherein the semiconductor integrated circuit includes a phase lockedloop circuit for generating the transmission signal, wherein the phaselocked loop circuit comprises: a voltage controlled oscillator circuitwhich generates an oscillating signal; a phase comparator circuit whichcompares the oscillating signal with a reference signal in phase andwhich generates both a first signal and a second signal in accordancewith a difference in phase between the oscillating signal and thereference signal; a first charge pump circuit which includes a firstcharging current source controlled by the first signal and a firstdischarging current source controlled by the second signal; a secondcharge pump circuit which includes a second charging current sourcecontrolled by the second signal and a second discharging current sourcecontrolled by the first signal, the second charging current source andthe second discharging current source generating currents whose absolutevalues are smaller than those of currents generated by the firstcharging and the first discharging current sources; and a loop filterincluding a charge/discharge node, a first capacitive element coupled tothe charge/discharge node, and a second capacitive element coupled tothe charge/discharge node through a first resistive element, wherein thefirst capacitive element is charged and discharged by the first chargepump circuit, and the second capacitive element is charged anddischarged by the second charge pump circuit, wherein the loop filterfurther comprises a second resistive element and a switching elementwhich are connected together in series, one terminal of the seriesconnection of the second resistive element and the switching element isdirectly coupled to the charge/discharge node, and the series connectionof the second resistive element and the switching element is connectedin parallel with the first resistive element.
 3. A radio communicationsystem according to claim 2, wherein the first capacitive element andthe second capacitive element are set to have capacitance values on thesame order.